Technical Field
The present disclosure relates to a communication device and a communication method that are, for example, applicable to a system in which wireless communication devices autonomously synchronize with each other to define communication timings for exchanging data.
Related Art
For example, nodes (wireless communication devices) configuring a sensor network are configured to perform power-saving communication. A representative method for power-saving communication is a method in which a reception node intermittently operates. For example, CSL (Coordinated Sampled Listening) is a communication standard employing the reception node intermittent operation method for the reception node (see Chapter 5. 1. 11. 1 in the IEEE Std 802. 15. 4e-2012).
FIG. 5 is a timing chart illustrating an outline of data communication operations in CSL.
An CSL reception period (CSL Period), a maximum CSL reception period (MaxCSL Period), and a startup time (also called “wakeup frame reception period”) are previously set for each node. The maximum CSL reception period is the maximum time for repeatedly transmitting wakeup frames (maximum wakeup frame transmission time) when seen from a transmission node. The startup time is a reception standby time in which a reception node is waked up.
A transmission node S that transmits the data first performs a data transmission operation in an asynchronous communication mode. The operation in the asynchronous communication mode is as follows. The transmission node S repeatedly transmits wakeup frames to the reception node R during the maximum CSL reception period. A wakeup frame includes a rendezvous time (RZTime) indicating the remaining time until data transmission. The rendezvous time decreases each time the wakeup frame is transmitted. The transmission node S repeatedly transmits the wakeup frames during the maximum CSL reception period, and then transmits a data frame to the node R.
The node R enters the reception standby state only in the wakeup frame reception period within one CSL reception period, and enters the sleep state in which the node R does not perform the reception operation in other periods.
In a case in which the node R receives a wakeup frame WU3 from the node S in the reception standby state W2, the node R acquires the rendezvous time in the frame WU3. Then, the node R stops the reception operation until right before the rendezvous time, and enters the reception standby state W3 immediately before the rendezvous time. Accordingly, the node R receives a data frame D1 from the node S.
In a case in which the node R receives the data frame D1, the node R returns a reception acknowledge frame ACK1 to the node S. Note that, an CSL reception period, and an CSL phase indicating a mismatch in the reception timing (a mismatch between the reception standby states W2 and W3, and the CSL reception period) are inserted into the reception acknowledge frame ACK1. In other words, the CSL phase indicates a mismatch between a synchronization information time and a reception time in the wakeup frame, and is a correction amount (synchronization correction amount) designated by the reception node R side, in order to keep synchronization with the reception node R at the transmission node S side.
In a case in which the node S has not received the reception acknowledge frame ACK1 from the node R, the node S repeatedly transmits the wakeup frames to the node R again in the asynchronous communication mode, also during next data transmission, and then transmits the data.
In a case in which the node S receives the reception acknowledge frame ACK1 from the node R, the node S enters the synchronous transmission mode. The node S holds the CSL reception period and the CSL phase included in the reception acknowledge frame ACK1, and corrects the synchronization with the node R. The node S then transmits a wakeup frame WU6 with a corrected transmission timing, during next data transmission to the node R. The wakeup frame WU6 is transmitted in consideration with a difference in clock accuracy between the timers of the two nodes S and R while the synchronous transmission mode is established, and the transmission is performed only in a shorter period ST than the maximum CSL reception period (Note that FIG. 5 illustrates that only one frame is transmitted, but plural frames may be repeatedly transmitted during the short period ST. The short period ST will be called below “wakeup frame transmission period”).
In a case in which a reception acknowledge frame ACK is not returned from the reception node R even in the synchronous communication mode, the transmission node S transits to the asynchronous communication mode, and performs the transmission operation in the asynchronous communication mode during next data transmission.
However, in the conventional communication device, the power saving effect and the synchronization accuracy are in a trade-off relationship, since an internal clock (timer) of each node temporarily synchronizes and then updates a time by its own generated clock. Namely, power saving in the reception node R becomes more effective, as the wakeup frame reception period becomes shorter. On the other hand, in a case in which the wakeup frame reception period is shortened, receiving of the wakeup frames becomes difficult, due to a synchronization mismatch between the internal clocks of the two nodes, or the like.
Each time a data frame is communicated in the synchronous communication mode, the transmission node and the reception node are synchronized with each other, and thus, synchronization mismatch is eliminated. However, in a case in which a new data frame is to be transmitted after long time has elapsed from the previous data frame transmission, a temporal difference caused by synchronization mismatch increases, and thus, a case in which the reception node cannot receive the wakeup frame prior to transmitting a next data frame may occur.
Hereinafter, with reference to FIG. 6, a case in which a wakeup frame cannot be received prior to transmission of a next data frame due to synchronization mismatch, will be described.
A wakeup frame transmission period (note that, the wakeup frame transmission period is not a maximum wakeup frame transmission time) is assumed as 100 milliseconds. (1) of FIG. 6 illustrates a case in which the transmission node S is repeatedly transmitting five wakeup frames WU1 to WU5 during the wakeup frame transmission period. In consideration of synchronization mismatch, it is preferable that the startup time in the reception node R is caused in the middle of the wakeup frame transmission period (at a timing when about 50 milliseconds has elapse from the start time). Further, the transmission node S is assumed to transmit a new data frame after one hour from the previous data frame transmission.
(2) of FIG. 6 illustrates a case in which the internal clock of the reception node R is faster than the internal clock of the transmission node S. Namely, (2) of FIG. 6 illustrates the case in which the internal clock of the reception node R has gained 50 milliseconds faster than the internal clock of the transmission node S during one hour, after the transmission node S has transmitted the data frame previously. In a case in which the two internal clocks completely synchronize with each other, the reception node R enters the wakeup frame reception period (the startup time) when the reception node R can receive the wakeup frame WU3. However, in a case in which the internal clock of the reception node R is 50 milliseconds faster than the internal clock of the transmission node S, the reception node R enters the wakeup frame reception period when the reception node R can receive the first wakeup frame WU1, and thus, receives the wakeup frame WU1. If the internal clock of the reception node R is much faster than the internal clock of the transmission node S, the reception node R cannot receive any of the wakeup frames WU1 to WU5, and transits to the asynchronous communication mode. In the example of FIG. 6, in a case in which the internal clock of the reception node R is over 50 milliseconds faster than the internal clock of the transmission node S, the reception node R cannot receive any of the wakeup frames WU11 to WU15, and transits to the asynchronous communication mode.
(3) of FIG. 6 illustrates a case in which the internal clock of the reception node R is lose than the internal clock of the transmission node S. Namely, (3) of FIG. 6 illustrates the case in which the internal clock of the reception node R has lost about 50 milliseconds behind the internal clock of the transmission node S during one hour, after the transmission node S has transmitted the data frame previously. In a case in which the internal clock of the reception node R is about 50 milliseconds behind than the internal clock of the transmission node S, the reception node R enters the wakeup frame reception period when the reception node R can receive the last wakeup frame WU5, and thus receives the wakeup frame WU5. In a case in which the internal clock of the reception node R is much behind than the internal clock of the transmission node S, the reception node R cannot receive any of the wakeup frames WU1 to WU5, and transits to the asynchronous communication mode. In the example of FIG. 6, in a case in which the internal clock of the reception node R is over 50 milliseconds behind than the internal clock of the transmission node S, the reception node R cannot receive any of the wakeup frames WU1 to WU5, and transits to the asynchronous communication mode.
As illustrated in FIG. 6, for example, in a case in which five wakeup frames are transmitted within 100 milliseconds, the rendezvous time is performed in units of about 20 milliseconds. In this way, in the CSL, there may be cases in which accurate synchronization control cannot be achieved due to coarse time units.
As described above, in the conventional communication device, there may be cases in which secure synchronization between the transmission node and the reception node cannot be achieved, when a data frame transmission interval is long. For example, in a sensor network or the like, a communication interval between the two nodes is longer, and synchronization may not be achieved in a case in which a new data frame is communicated, and consequently, a large number of wakeup frames are transmitted in order to transit to the asynchronous communication mode.
Accordingly, a communication device, a communication device synchronization method, a communication program, and a communication system that may keep synchronous communication even with a long data frame transmission interval are desired.